Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefins such as ethylene and/or propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in many processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. This process is referred to as the oxygenate-to-olefin process. The preferred oxygenate for light olefin production is methanol. The process of converting methanol-to-olefins is called the methanol-to-olefins process.
There are numerous technologies available for producing oxygenates, and particularly methanol, including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. The most common process for producing methanol is a two-step process of converting natural gas to synthesis gas. Then, synthesis gas is converted to methanol.
Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Synthesis gas production processes are well known, and include conventional steam reforming, autothermal reforming or a combination thereof.
Synthesis gas is then processed into methanol. Specifically, the components of synthesis gas (i.e., hydrogen, carbon monoxide and/or carbon dioxide) are catalytically reacted in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one process, methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
The methanol is then converted to olefins in a methanol-to-olefins process. The methanol-to-olefins reaction is highly exothermic and has a large amount of water. Water comprises as much as one half of the total weight of the output stream of the reactor or effluent stream. Consequently, the water must be removed by condensation in a quench device to isolate the olefin product. A quench device cools the effluent stream to the condensation temperature of water. Quenching the product recovers large quantity of water at the temperature near the boiling point of the quench medium.
Oxygenate-to-olefins reactions, including methanol-to-olefin reactions, use a catalyst, preferably a molecular sieve catalyst. Catalyst particles are circulated through the reactor system and are retained by the particle size separators or cyclones. The catalyst particles break down into smaller particles as they make physical contact with the hardware of the reactor system and pass through the particle size separators or cyclones into the effluent stream. As used herein, any catalyst particles that are found in the effluent stream are referred to as catalyst fines. After passing through the cyclones, catalyst fines are carried along in the effluent stream. Typically, the particle size separators retain catalyst particles that are above 40 microns in size. Catalyst particles are removed from the effluent stream in a quench device, and typically are suspended in a bottoms stream of the quench device. The presence of catalyst fine in the effluent stream or the bottoms stream of a quench device presents two unique problems. First, the catalyst fines can erode the walls of the vessel as they travel through the reactor. Second, catalyst fines can settle or collect in parts of the equipment. This settling or accumulation can damage equipment, cause inefficient operation, shut downs and frequent cleaning.
U.S. Pat. No. 6,121,504 describes a quench apparatus for an oxygenate-to-olefins process as well as a process for using a quench apparatus. The process removes water from the effluent stream as well as some oxygenate feedstock such as methanol.
U.S. Pat. No. 6,403,854 describes a two stage solids wash and quench for use with the oxygenate conversion process where catalyst fines are removed from the effluent stream through a first quench stage. Water and methanol are removed from the effluent stream in a second quench stage. The quench bottoms from the first quench stage are withdrawn as an aqueous waste stream or drag stream and are sent to a water treatment zone.
It would be desirable if there was a process for operating an oxygenate-to-olefin plant to avoid these problematic consequences. The present invention satisfies these and other needs.